This invention relates generally to photodrafting equipment, that is to say, equipment arranged to project a beam of light to illuminate a spot on a photosensitive surface and to move the spot of light relatively to the surface to expose a line thereon. More particularly, the invention relates to projection apparatus for photodrafting equipment.
Photodrafting equipment is commonly used to expose photographic sheets used in turn to produce masks for the production of printed circuit boards and the like. Projection apparatus mounted on a carriage movable in X- and Y-co-ordinate directions under the control of a computer or other control means is arranged to project a beam of light substantially orthogonally against a photographic sheet disposed in a plane parallel with the X- and Y-co-ordinate directions. By this means, a line can be exposed on the sheet by movement of the carriage during continuous projection of the beam, or discrete "pads" can be exposed in selected positions on the sheet by moving the carriage to the corresponding co-ordinate positions successively, the light beam being projected only when the carriage is located at one of those positions.
A problem which arises when using the equipment for the exposure of discrete pads, especially if large numbers of such pads are included in the pattern to which the sheet is to be exposed, relates to the time taken in moving the carriage from one exposure position to the next. The times required to accelerate the carriage to maximum speed from standstill at one exposure position and then to decelerate the carriage to standstill again at the next exposure position can form a substantial proportion of the total time taken for the exposure of a complete pattern. Accordingly, reduction or elimination of the acceleration and deceleration times could provide a substantial increase in the production capacity of the equipment.